By definition, federated search involves the simultaneous search of multiple databases or collections.
Federated search may be necessary when managing dynamic content from multiple sources with different data models, but it does present challenges.
First and foremost, performance is notoriously slow. As the number and size of collections grow, speed suffers. This is a serious flaw.
Plus, the query language may be limited to the lowest common denominator in the face of disparate vocabularies and data models. Sophisticated approaches like faceted navigation may be precluded by federated search.